Passive saturable absorbers have been extensively investigated for use in laser systems to extend the bit-rate and error-free transmission of periodically amplified optical transmission systems. At each amplification stage the optical signal is regenerated. Saturable absorber devices are simple and cost effective devices for passive optical regeneration. The noise suppression capability of such devices can attenuate the accumulated amplified spontaneous emission noise more than the higher-power signal component, thereby increasing the signal-to-noise ratio.
One common category of saturable absorbers considered for practical commercial applications are semiconductor devices. Semiconductor saturable absorber devices involve relatively complex and costly fabrication methods. This adds complexity and cost to the system. Moreover, they operate in a reflection mode. A saturable absorber device operating in a transmission mode is more desirable for many applications, notably for in-line elements in an optical fiber system. Optical saturable absorbers operating in transmission have been the focus of recent development efforts in this field.
New studies have shown that carbon nanotubes, typically single-walled carbon nanotubes (SWNTs), exhibit effective passive saturable absorption when placed in the optical path of a laser beam. See for example, S. Y. Set et al., Laser Mode-Locking and Q-Switching Using a New Saturable Absorber Material Based on Carbon Nanotubes, and Yamashita et al., Saturable absorbers incorporating carbon nanotubes directly synthesized onto substrates and fibers and their application to mode-locked fiber lasers. Both of these papers are incorporated herein by reference.
Techniques studied so far for preparing optical fiber-based SWNT saturable absorber devices are primarily SWNT growth methods wherein the surface of the optical fiber to be coated with carbon nanotubes is catalyzed with a metal catalyst, and carbon nanotubes are grown on the catalyzed surface. This method is essentially a catalyzed Chemical Vapor Deposition (CVD) method. While CVD and similar growth methods have proven useful, new techniques for simple, cost-effective, manufacture of optical fiber-based SWNT saturable absorber devices are in demand.